Alloy

ABSTRACT

An iridium alloy consists essentially of iridium and at least one of W and Zr, and optionally Rh. When present, W comprises between 0.01 and 5 wt % of the alloy; when present in combination with W, Zr comprises between 0.01 and 0.5 wt % of the alloy; when present alone or in combination with Rh only, Zr comprises between 0.01 and 0.09 wt % of the alloy; and when present, Rh comprises between 0.1 and 5 wt % of the alloy. The alloys may be modified by the addition of platinum and other platinum group metals and base metals. The alloys demonstrate enhanced physical and chemical properties and are suitable for use as electrode materials in spark plugs and other high temperature applications.

This invention relates to alloys of iridium, in particular to alloys ofiridium with low amounts of alloying elements and uses thereof.

Iridium is a member of the platinum group of metals and has a variety ofapplications including automobile catalysts, electrodes for industrialelectrolysis, crucibles for crystal growth, thermocouples, rocket motorparts, glass making and spark plugs. It has several attractiveproperties including a very high shear modulus at room temperature andelevated temperature strength second only to tungsten among therefractory metals. It is also thought to be the most corrosion resistantof all metals.

However, despite these benefits there are some disadvantages. Itsmechanical properties are sensitive to certain low level impurities andstrain rate and it also exhibits a ductile-brittle transition. Due tothe rarity of its occurrence in nature its price per gram is of the sameorder as platinum and furthermore its density is the second highest ofall elements. Finally, although compared to the refractory metals itsresistance to oxidation is excellent, it nevertheless does exhibit asignificant weight loss at elevated temperature under oxidisingconditions.

As a result of its scarcity and difficulty in maintaining metal purityduring manufacture, the metallurgy of iridium is poorly understood.Indeed little work, relative to that done on alloying of platinum forinstance, has been carried out to investigate the effect of alloying onproperties. However, the alloying with some elements has beeninvestigated by different workers. Oak Ridge National Laboratories inthe USA have been responsible for developing one alloy, DOP-26 based onIr—0.3 W+Th, for radioisotope thermoelectric generator casings used tosupply power to spacecraft. Tungsten has been shown to increase thealloy re-crystallisation temperature of iridium by 400° C. at >2 wt %addition, which makes control of microstructure during hot working, muchsimpler. Thorium has been shown to promote ductility below the normalductile/brittle transition zone, although its radioactivity is a majordisadvantage when considering this alloy for normal commercialapplications. Certain Rare Earth elements, Ce, Y and Lu have also beeninvestigated, and Ce has been found promote similar properties to Th,although less pronounced. ORNL have developed a new alloy range based onIr—0.3 W with low levels of Ce+Th.

U.S. Pat. No. 3,918,965 describes a binary alloy of iridium with 0.3 to1 wt % hafnium. Improvements in physical properties are claimed.

Work has been limited in respect of alloying iridium with platinum groupmetals (PGM). Rhodium additions, up to a maximum of ca. 10 wt %, havebeen shown to improve oxidation resistance, ductility and formability.Application of 40% Rh—Ir to novel rocket nozzles was reported in theearly 1990's. Ternary alloys have also been long considered for pennibs, and electrodes. The advent of long life spark plugs hasre-invigorated interest in the potential of iridium alloys. Rhodiumadditions have been found to be beneficial, with 40wt % being best foroxidation resistance. Additions of 10 wt % of both platinum andpalladium also improve the oxidation resistance of iridium, although notas effectively as rhodium. Al, Si, Cr, Mo and W were found to beineffective.

EP0866530 A1 discloses ternary and quaternary alloys of iridium, rhodiumand at least one of rhenium and ruthenium. Low levels of Re and Ru,either singly or combined, significantly reduce the oxidation loss of analloy at 1100° C. for 30 hours, compared to pure iridium. The presenceof rhodium is essential, as Re and Ru have little or no effect whencombined with iridium alone.

JP 2000290739 A discloses an alloy for the formation of crucibles whichcan be used at high temperatures without significant deformation oroxidation. The alloy is a binary or ternary alloy of iridium with0.5-40wt % of Rh and/or Pt.

JP 10259435 A discloses a heat resistant iridium alloy which comprises abase of iridium to which 0.1 to 50 wt % of one or more secondaryelements is added. Platinum, palladium, rhodium, niobium, tantalum,hafnium, titanium, zirconium, yttrium and lanthanum are suggested assecondary elements however actual examples of only some of these aregiven, none of which contain secondary elements at less than 1 wt %.

U.S. Pat. No. 3,070,450 discloses alloys formed from a base of pureiridium or iridium—0.3 wt % W, to which small amounts each of aluminium,iron, nickel, rhodium and thorium are added. The alloys are useful forthe encapsulation of radioactive sources so the use of thorium can betolerated. Thorium containing alloys are not usually suitable forgeneral application.

U.S. Pat. No. 3,293,031 discloses a ductile ternary iridium alloycontaining up to 0.5 wt % of both titanium and zirconium.

Although prior attempts to improve the physical and mechanicalproperties of iridium by alloying have met with some success, thereremains a need for further improvements.

In accordance with the present invention, an iridium alloy consistsessentially of iridium, at least one of W and Zr and optionally Rh;wherein when present, W comprises between 0.01 and 5 wt % of the alloy;wherein when present in combination with W, Zr comprises between 0.01and 0.5 wt % of the alloy; wherein when present alone or in combinationwith Rh only, Zr comprises between 0.01 and 0.09 wt % of the alloy; andwherein when present, Rh comprises between 0.1 and 5 wt % of the alloy.

Preferably, when present, W comprises between 0.01 and 0.5 wt % of thealloy; when present in combination with W, Zr comprises between 0.01 and0.5 wt % of the alloy; and when present alone or in combination with Rhonly, Zr comprises between 0.02 and 0.07 wt % of the alloy.

It will be understood that whilst the amounts of each component aregiven assuming that the base alloy is pure iridium, in practical terms,the iridium and the alloying elements may contain impurities at levelswhich would normally be expected for such metals.

The alloys of the present invention show enhanced physical andmechanical properties over pure iridium.

The alloy of the present invention may be modified by the addition of Ptin an amount of between 0.1 and 5 wt % of the alloy.

Additionally or alternatively, the alloy of the present invention may bemodified by the addition of one or more of Ta, Nb, Mo, Cr, Ce, Sc, Lu,Co, Ni, Hf, Y, Ti, Ru and Pd individually in an amount of between 0.01and 10 wt % of the alloy.

Preferably, when present, Ta, Nb, Mo, Cr, Ce, Sc, Lu, Co, Ni, Hf; Y andTi individually comprise between 0.01 and 0.5 wt % of the alloy; andwhen present, Ru and Pd individually comprise between 0.1 and 5 wt % ofthe alloy.

In a preferred embodiment, the alloy consists essentially of iridium, Wand Zr.

In a further preferred embodiment, the alloy consists essentially ofiridium and W.

In a yet further preferred embodiment, the alloy consists essentially ofiridium and Zr.

In measurements of stress rupture times at elevated temperatures, thesealloys may outperform pure iridium by a factor of twenty or more. Creeprates at high temperature are also significantly reduced. Furthermore, Wand Zr may also retard grain growth at high temperature, with smalladditions of both W and Zr being found to reduce the rate of graingrowth at high temperature by a factor of two compared to pure iridium.

In a yet further preferred embodiment, the alloy consists essentially ofiridium, Rh, W, and Zr.

In a yet further preferred embodiment, the alloy consists essentially ofiridium, Pt, Rh, W and Zr.

Significant reduction in weight loss under high temperature oxidisingconditions is found for these alloys, when compared to pure iridium.

In a yet further preferred embodiment, the alloy consists essentially ofiridium, Rh and W.

In a yet further preferred embodiment, the alloy consists essentially ofiridium, Rh and Zr.

In a yet further preferred embodiment, the alloy consists essentially ofiridium, Pt, Rh and W.

In a yet further preferred embodiment, the alloy consists essentially ofiridium, Pt and W. In tensile tests, these alloys demonstrate aconsiderable increase in elongation to failure compared to pure iridium.In some cases, elongation to failure is increased two-fold and more.

The enhanced physical and mechanical properties of the alloys of thepresent invention make them suitable for use in many high temperature orload bearing applications. For example, they may be used in ignitionapplications i.e. as components in spark-plugs or as crucibles, e.g. forcrystal growing or other equipment in chemical and glass applicationswhere high strength, low creep rate and good oxidation resistance arerequired. Other applications include electrodes, heat shields and rocketnozzles. The foregoing examples merely serve to illustrate the manypotential uses of the present alloys, and as such, are not intended tobe limiting in any way.

The alloys may be manufactured by known methods and fabricated into anysuitable physical form. Improvements in elongation to failure, orductility, make the alloys particularly suitable for drawing into wireshowever, tubes, sheets, grains, powders or other common forms are alsocontemplated. The alloys may also be used in spray coating applications.

The invention will now be described by way of example only and withreference to the following drawings in which;

FIG. 1 is a bar chart comparing the mean elongation at room temperatureof an alloy according to the present invention with pure iridium;

FIG. 2 is a bar chart comparing the stress rupture time at elevatedtemperature of four alloys according to the present invention with pureiridium;

FIG. 3 is a bar chart comparing the rate of grain growth at elevatedtemperature of four alloys according to the present invention with pureiridium;

FIG. 4 is a graph comparing the measured weight loss of two alloysaccording to the present invention with pure iridium, and;

FIG. 5 is a bar chart comparing the oxidation rate at two temperaturesof several alloys according to the present invention with commercialiridium alloys.

EXAMPLE 1 Alloy Preparation

The alloys detailed in table 1 below were prepared by argon arc melting.All values are given in weight percent based on the total weight of thealloy. Balance in all cases is iridium. TABLE 1 Alloy W Zr Rh Pt Other 1 0.3 — — 0.2 —  2 — 0.07 — — —  3 0.3 0.02 — — —  4 0.05 — — — —  50.02 0.02 — — —  6 0.3 0.07 2.5 — —  7 0.3 0.07 2.5 2.5 —  8 0.3 — 2.52.5 —  9 0.5 — 1.0 — — 10 0.3 — 1.0 1.0 — 11 0.3 — 1.0 5.0 — 12 1.0 —1.0 — — 13 2.0 — 2.5 — — 14 0.5 — 2.5 — — 15 — 0.07 2.5 — — 16 0.3 — — —— 17 — — 2.5 — Ta (0.5) 18 — — 2.5 — Nb (0.25) 19 — — 2.5 — Mo (0.25) 20— — 2.5 — Cr (0.15) 21 — — 2.5 — Pd (0.3) 22 0.05 — — 5.0 — 23 0.05 —0.5 5.0 — 24 0.3 — 5.0 1.0 —

EXAMPLE 2 Elongation to Failure

Alloy 1 was hot drawn into wires of 1.8 mm diameter, and subjected totensile testing with a gauge length of 51 mm and a cross head speed of 5mm/minute. The result is shown in FIG. 1. Addition of Pt and W at theppm level significantly improved the room temperature mechanicalproperties of the alloy. Although ultimate tensile strength was found toonly be improved marginally, elongation to failure increased by 117%relative to similar wires of pure iridium.

EXAMPLE 3 Stress Rupture

Alloys 2-5 were hot rolled into sheets and tensile sample blanks formedby spark erosion machining. These were then surface ground to athickness of nominally 1.8 mm. The gauge length of each sample blankswas 30 mm. Stress rupture times were measured at a temperature of 1400°C. and stress of 75 MPa Results are shown in FIG. 2. Significantimprovements in stress rupture times were found for all alloys comparedto pure iridium, with ppm levels of Zr (alloy 2) or Zr and W (alloy 5)being most effective. Although not shown in FIG. 2, creep rates atelevated temperature were also reduced, in some cases by as much as afactor of 16 compared to pure iridium.

EXAMPLE 4 Grain Growth Retardation

Alloys 2-5 as detailed in table 1 above, were hot rolled into sheet ofnominally 3.5 mm thickness. The alloys were held at 1550° C. for 400hours and grain size measurements made. This was done using an opticalmicroscope. The number of grains intersecting a line traversing thepolished and etched section were counted and averaged over the crosssectional thickness. Results are shown in FIG. 3. Grain growth wasreduced for all alloys compared to pure iridium, with ppm levels of Zrand W (alloy 5) showing a halving of grain size.

EXAMPLE 5 Oxidation Weight Loss

Alloys 6 and 7, as detailed in table 1 above, were hot drawn into wiresof between 0.6 and 1.2 mm and their weights monitored while being heldat 1000° C. for 200 hours. Results are shown in FIG. 4. The weight lossof both alloys was approximately 4 times less than that for pureiridium, over the duration of the test, and approached that which wasfound for a commercial 10 wt % Rh—Ir alloy.

Further oxidation weight loss experiments were carried out using wiresof different thicknesses formed from alloys according to the presentinvention. FIG. 5 shows the weight loss rates of alloys 1, 4, 5, 13, 14and 15. The heavily shaded bars in FIG. 5 represent experiments carriedout at 1000° C. and the lighter shaded bars represent experimentscarried out at 1100° C. The figure in brackets refers to the thicknessof the wire in mm. Oxidation rate is expressed in g/mm·hour. All alloysshowed a significant reduction in oxidation rate compared to a 5% Pt—Iralloy.

EXAMPLE 6 Engine Tests

Alloys 6 and 7, as detailed in table 1 above, were formed into sparkplug electrodes. During testing in a high performance car engine over aperiod of 175 hours, the electrodes were found to erode at a similarrate to commercial 10 wt % Rh—Ir alloy electrodes, and at a much reducedrate compared to pure iridium electrodes.

1. An iridium alloy, consisting essentially of iridium, Rh and at leastone of W and Zr; wherein the Rh comprises between 0.1 and 2.5 wt % ofthe alloy; wherein when present, W comprises between 0.01 and 5 wt % ofthe alloy; wherein when present in combination with W, Zr comprisesbetween 0.01 and 0.5 wt % of the alloy; and wherein when present incombination with the Rh only, Zr comprises between 0.01 and 0.09 wt % ofthe alloy.
 2. An iridium alloy according to claim 1, wherein whenpresent, W comprises between 0.01 and 0.5 wt % of the alloy; and whereinwhen present in combination with the Rh only, Zr comprises between 0.02and 0.07 wt % of the alloy.
 3. An alloy comprising an iridium alloyconsisting essentially of iridium, Rh and at least one of W and Zr;wherein the Rh comprises between 0.1 and 2.5 wt % of the alloy; whereinwhen present, W comprises between 0.01 and 5 wt % of the alloy; whereinwhen present in combination with W, Zr comprises between 0.01 and 0.5 wt% of the alloy; and wherein when present in combination with the Rhonly, Zr comprises between 0.01 and 0.09 wt % of the alloy, modified bythe addition of Pt in an amount of between 0.1 and 5 wt % of the alloy.4. An alloy comprising an iridium alloy consisting essentially ofiridium, Rh and at least one of W and Zr; wherein the Rh comprisesbetween 0.1 and 2.5 wt % of the alloy; wherein when present, W comprisesbetween 0.01 and 5 wt % of the alloy; wherein when present incombination with W, Zr comprises between 0.01 and 0.5 wt % of the alloy;and wherein when present in combination with the Rh only, Zr comprisesbetween 0.01 and 0.09 wt % of the alloy, modified by the addition of oneor more of Ta, Nb, Mo, Cr, Ce, Sc, Lu, Co, Ni, Hf, Y, Ti, Ru and Pdindividually in an amount of between 0.01 and 10 wt % of the alloy. 5.An alloy according to claim 4, wherein when present, Ta, Nb, Mo, Cr, Ce,Sc, Lu, Co, Ni, Hf, Y and Ti individually comprise between 0.01 and 0.5wt % of the alloy; and wherein when present, Ru and Pd individuallycomprise between 0.1 and 5 wt % of the alloy.
 6. An iridium alloyaccording to claim 1, the alloy consisting essentially of iridium, Rh,W, and Zr.
 7. An iridium alloy according to claim 3, the alloyconsisting essentially of iridium, Pt, Rh, W and Zr.
 8. An iridium alloyaccording to claim 1, the alloy consisting essentially of iridium, Rhand W.
 9. An iridium alloy according to claim 1, the alloy consistingessentially of iridium, Rh and Zr.
 10. An alloy according to claim 3,the alloy consisting essentially of iridium, Pt, Rh and W.
 11. Anelectrode comprising an iridium alloy according to claim
 1. 12. A sparkplug comprising an electrode according to claim
 11. 13. An iridium alloyaccording to claim 3, wherein when present, W comprises between 0.01 and0.5 wt % of the alloy; and wherein when present in combination with theRh only, Zr comprises between 0.02 and 0.07 wt % of the alloy.
 14. Aniridium alloy according to claim 4, wherein when present, W comprisesbetween 0.01 and 0.5 wt % of the alloy; and wherein when present incombination with the Rh only, Zr comprises between 0.02 and 0.07 wt % ofthe alloy.
 15. An alloy according to claim 4, further comprising Pt isin an amount of between 0.1 and 5 wt % of the alloy.
 16. An iridiumalloy according to claim 2, the alloy consisting essentially of iridium,Rh, W, and Zr.
 17. An iridium alloy according to claim 2, the alloyconsisting essentially of iridium, Rh and W.
 18. An iridium alloyaccording to claim 2 the alloy consisting essentially of iridium, Rh andZr.